Typically a gradient system is made of three axes, X, Y and Z, this means that the system can supply three different gradient pulses on three different coils at the same time. A gradient supply for a given axis is also called a gradient channel.
It is known that the remaining noise level of the power sources feeding the gradient coils results in artefacts or other detrimental signals and thus leads to erroneous measurements or distorted images, in particular when high resolution experiments are involved.
A first type of solution is based on external hardware switching means, which when activated cut the gradient coils from their power source(s), in particular from the output stage of the feeding power amplifiers. One example of such a hardware switch is disclosed in EP 0 598 670.
A second type of solution has been proposed in relation to digitally controlled power feeding means, wherein a pulse program defines the pulse sequences which are specific to each NMR experiment.
A digital gradient pulse generator translates the programmed pulses into system wide synchronous digital gradient data pulses, in the form of word streams. A data dispatcher then extracts from the consecutive words the data for each channel and sends them to the channel's bipolar digital-to-analog voltage converter.
Finally, the gradient amplifier, which can be considered as a transconductance amplifier and constitutes a module with its associated converter, supplies the current to the specific axis coil. The reference current in the gradient coil (0 A) is generally given for an amplifier input of 0V (i.e. the digital mid-scale).
When implementing such digitally controlled power feeding means, a blanking signal code can be embedded into the adequate words of the word stream corresponding to the translated programmed pulses, said blanking signal allowing to disable the output of the concerned amplifier.
The gradient data words which are delivered by the digital gradient data generator generally comprise at least the following four fields:                a data field containing the gradient data to be converted by the digital-to-analog converter. As seen before, the data representation is normally bipolar and the mid-scale (zero) value is the reference value,        a channel address field containing the address of the gradient channel for which the data are intended,        a strobe flag (which is used to signalize that the data and address fields are valid),        a next gradient order flag        
Furthermore, these gradient data words can be of three different types, namely:                Effective data words: for these words, the data, address and the strobe flag are valid, the next gradient order flag is disabled. When this word is received, the data is latched for the specified gradient channel.        Next Gradient Order words: for these words, the next gradient order flag is raised and the data, address and the strobe fields are discarded. When this word is received the last latched data for each gradient channel is send to the converter.        Stuffing words: for these words, the strobe and next gradient order flags are not raised. Such kind of words have no effect on the gradient output and have to be discarded.        
It should be noticed that the words are transmitted synchronously and that therefore a constant reference clock signal is transmitted with the data lines. Generally the pulse generator sends much more stuffing words than the two other word types. This implies that the word rate itself can be much higher than the actual gradient data rate.
When applying the second type of solution, the blanking signal has to be enabled when the amplifier's input, for a given channel, is set to 0 volt (or to a value resulting in no current being fed to the connected gradient coil).
Nevertheless, some hardware linked constraints must be observed when switching the blanking signal (see FIG. 1 showing [the gradient data/blanking signal] timing chart):                Because of the amplifier output stage activation delay, when the gradient data go from a null value to a non null value (i.e. the gradient supply is to be enabled), the blanking signal must be disabled with a given delay (T1 on FIG. 1) before the gradient data becomes non null.        Because of the time constants introduced by the amplifier frequency response and the load (the gradient coil), when the gradient data switch from a non null to a null value (i.e. the gradient supply is to be disabled), the blanking signal must be enabled with a given delay (T2 on FIG. 1) after the data were driven to zero.        
As a consequence of the two preceding points, the blanking signal must not be activated in case of a “data is null” sequence which does not last longer than the sum of T1 and T2. T1 and T2 have generally the same values for all gradient channels, but are dependent on the hardware of the gradient channels.